1. Field of the Invention
The present invention relates to a method of driving a solid-state image pickup apparatus that transfers charges between registers.
2. Description of the Prior Art
FIG. 1 shows a solid-state image pickup apparatus that transfers charges between registers according to the prior art.
This apparatus has a line of photosensors 101. The photosensors 101 are arranged in a main scanning direction and generate signal charges in response to the intensity of incident light. Shift gates 111 and 112 are alternately arranged along the photosensors 101. The shift gates 111 control the integration period of odd ones of the photosensors 101, and the shift gates 112 control the integration period of even ones of the photosensors 101.
Inner and outer analog shift registers 121 and 122 are arranged in parallel with the shift gates 111 and 112. The inner register 121 has transfer electrodes .o slashed. 1 adjacent to the shift gates 111 and transfer electrodes .o slashed. 2 adjacent to the shift gates 112. Similarly, the outer register 122 has transfer electrodes .o slashed. 1 and .o slashed. 2 that are alternated. The electrodes .o slashed. 1 of the inner and outer registers 121 and 122 receive a drive pulse, and the electrodes .o slashed. 2 thereof receive another drive pulse. Namely, they are driven in two phases.
Transfer gates 131 are arranged between the electrodes .o slashed. 1 of the inner and outer registers 121 and 122, for controlling the transfer of charges between the registers 121 and 122. An output circuit 141 converts charges transferred by the inner register 121 into a voltage, and an output circuit 142 converts charges transferred by the outer register 122 into a voltage.
A method of driving the apparatus of FIG. 1 will be explained with reference to a timing chart of FIG. 2.
An electrode SH1 of each shift gate 111 and each electrode .o slashed. 1 are opened, i.e., set to HIGH at time t1 to transfer signal charges from the odd photosensors 101 to the electrodes .o slashed. 1 of the inner register 121. At this time, dark charges in the electrodes .o slashed. i are mixed with the signal charges. Since the electrodes .o slashed. 2 are set to LOW, dark charges In the electrodes .o slashed. 2 are transferred to the electrodes .o slashed. 1, and these are mixed with each other.
An electrode TG of each transfer gate 131 is opened at time t2, to transfer the signal charges mixed with the dark charges of the electrodes .o slashed. 1 and .o slashed. 2 are transferred from the inner register 121 to the electrodes .o slashed. 1 of the outer register 122 at time t3. After all charges of the inner register 121 are transferred to the outer register 122, an electrode SH2 of each shift gate 112 is opened at time t4, to transfer signal charges from the even photosensors 101 to the electrodes .o slashed. 2 of the inner register 121.
After time t5, charges are transferred from the electrodes .o slashed. 2 to .o slashed. 1 when the electrodes .o slashed. 1 are HIGH and the electrodes .o slashed. 2 are LOW. When the electrodes .o slashed. 1 are LOW and the electrodes .o slashed. 2 are HIGH, charges are transferred from the electrodes .o slashed. 1 to the electrodes .o slashed. 2. Thereafter, the charges are successively transferred to the output circuits 141 and 142.
The problems of the prior art will be explained with reference to FIG. 3, in which (a) shows a waveform of the dark output of the inner register 121 provided through the output circuit 141, and (b) shows a waveform of the dark output of the outer register 122 provided through the output circuit 142. Each waveform includes a register-to-register transfer period K1, an output period K2 of the photosensors 101, and a transfer margin period K3.
The waveform (a) is zero in the period K1 (corresponding to a period between t2 and t3 of FIG. 2) because all dark charges of the inner register 121 are transferred to the outer register 122 in this period. The number of electrodes .o slashed. 1 and .o slashed. 2 that are present between one of the photosensors 101 and the output circuit 141 is different from photosensor to photosensor. Accordingly, dark charges in the electrodes .o slashed. 1 and .o slashed. 2 are accumulated to cause a shading, i.e., an inclination to the right in the dark output (a). A total of the accumulated dark charges of the inner register 121 is P1.
Referring to the dark output (b) of the outer register 122, dark charges in the inner register 121 are added to dark charges in the outer register 122 in the period K1 (corresponding to a period between t2 and t3 of FIG. 2). If the dark charges in the inner register 121 are not added, the dark charges in the outer register 122 will be steady. Since the dark charges transferred from the Inner register 121 to the outer register 122 have a shading to the left, the dark output (b) of the outer register 122 has a shading to the left. The dark charges in the outer register 122 are represented with P2.
FIG. 4 shows a waveform (a) of the bright output of the inner register 121 provided through the output circuit 141, and a waveform (b) of the bright output of the outer register 122 provided through the output circuit 142. These waveforms are formed by adding given output signals to the waveforms (a) and (b) of FIG. 3.
In FIG. 4, a reference mark P11 represents the output of the inner register 121 due to light, and P12 and P13 represent dark charges in the inner register 121. A reference mark P14 represents dark charges in the outer register 122, P15 represents the output of the outer register 122 due to light, and P16 represents dark charges transferred from the inner register 121.
As is apparent in FIG. 4, a bright output waveform is formed by adding a given output signal to a dark output waveform. Namely, the bright output waveform includes a shading that causes uneven brightness in an image.